As related in the '312 application, isobutene is widely used for the production of a variety of industrially important products, such as butyl rubber for example. Isobutene has been produced commercially to date through the catalytic or steam cracking of fossil feedstocks, and the development of a commercially viable process for the manufacture of isobutene from a renewable source-based feedstock has accordingly become of great interest as fossil resources are increasingly depleted and/or have become more costly to use—especially in consideration of increased demand for isobutene.
Previous to the earlier '433 application, a hard-template method had been described for synthesizing ZnxZryOz mixed oxides for the direct and high yield conversion of ethanol (from the fermentation of carbohydrates from renewable source materials, including biomass) to isobutene, wherein ZnO was added to ZrO2 to selectively passivate zirconia's strong Lewis acidic sites and weaken Brönsted acidic sites while simultaneously introducing basicity. The objectives of the hard template method were to suppress ethanol dehydration and acetone polymerization, while enabling a surface basic site-catalyzed ethanol dehydrogenation to acetaldehyde, an acetaldehyde to acetone conversion via aldol-condensation/dehydrogenation, and a Brönsted and Lewis acidic/basic site-catalyzed acetone-to-isobutene reaction pathway.
High isobutene yields were in fact realized, but unfortunately, as later experienced by Mizuno et al. (Mizuno et al., “One—path and Selective Conversion of Ethanol to Propene on Scandium-modified Indium Oxide Catalysts”, Chem. Lett., vol. 41, pp. 892-894 (2012)) in their efforts to produce propylene from ethanol, it was found that further improvements in the catalyst's stability were needed.
The '433 application concerned the discovery that these improvements could be realized without adding modifying metals and without a reduction in the initial high activity (100 percent ethanol conversion) that had been observed in these mixed oxide catalysts, while the '312 application concerned the further discovery that the mixed oxide catalysts we had been evaluating for converting ethanol to isobutene are also able to catalyze the conversion of acetic acid to isobutene. Since acetic acid can be made by a variety of methods from a number of different starting materials, including through carbonylation of methanol derived from sequestered carbon dioxide, for example, the capability of these mixed oxide catalysts to catalyze the conversion of acetic acid to isobutene enabled a range of options for utilizing renewable resources more efficiently, all as described in greater detail in the '312 application.
In one of these methods for making the acetic acid, namely, by fermentation from a source of five and/or six carbon sugars, propionic (or propanoic) acid is often produced in addition to acetic acid—and this is especially true where the five and/or six carbon sugars have been obtained from the hydrolysis of a lignocellulosic biomass. Most propionic acid manufactured today is used as a preservative in both animal feed as well as in food for human consumption, but extensive use of an acetic acid process according to the '312 application to meet present and foreseeable isobutene demand would certainly benefit from and perhaps require a higher value-added outlet for the propionic acid that can be co-produced with the acetic acid.